


Lock and Chain

by Askellie



Category: Undertale (Video Game), Underverse - Fandom
Genre: Abuse of Authority, Body Horror, Brainwashing, Dubious Consent, Emotional Manipulation, M/M, Master/Servant, Overstimulation, Ownership, Psychological Manipulation, Soul Touching, Tentacles, soul manipulation
Language: English
Status: Completed
Published: 2020-08-03
Updated: 2020-08-03
Packaged: 2021-03-06 07:53:29
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,234
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25689883
Author URL: https://archiveofourown.org/users/Askellie/pseuds/Askellie
Summary: Cross can't sleep, not with Chara clawing at the back of his mind like a rat in the walls, just waiting for a chance to break through.It takes all of Nightmare's influence to keep the human sealed away, and for a creature of darkness and negativity, he's surprisingly willing to help.
Relationships: Crossmare, Nightmare(Dreamtale)/Cross(X-Tale), Sans/Sans (Undertale)
Comments: 20
Kudos: 210
Collections: Undertale Smut





	Lock and Chain

Cross opens his eyes to a darkness so thick it feels like smoke, burning in his sockets and scratching at the back of his throat. The bitter air makes his throat rattle as he tries to draw a deeper breath only to be pulled up short by a constricting vise around his chest. His body is wrapped in chains that glow like red-hot embers, but instead of burning their touch against his bones is icy. He can feel them drawing out his strength, discouraging him from fighting against the bonds or the oppressive gloom.

He hates this dream.

Even though Cross knows to expect it, he still flinches as Chara’s hands clamp down on his shoulders. His blunt, soft, human fingers feel like knives biting cruelly into his collarbone.

“You lose, Cross,” Chara murmurs smugly against his skull. Their smile looks like a gory slash across their face. “Now I’m the one in control. Just go to sleep. You’re really tired, aren’t you?”

He is, he _always_ is, because falling asleep means being here again, facing down this inner demon and the horrifying sensation of being a prisoner inside his own body. He tries to ignore Chara’s mocking laughter, straining against the chain. It bites into his arms, digging in unpleasantly despite being only a phantom of real pain.

It is a dream. It’s not real. Chara is sealed, and all he has to do is--

“ _Wake up_.”

His back hits a flat surface, hard enough to jolt him but not enough to do any damage. Cross gasps, sockets flying open. He blinks rapidly at the sudden change in brightness, trying to adjust to his surroundings. The room -- his room, stark and plainly furnished but comfortingly familiar -- is lit only by the muted glow of a lamp, but it almost feels like high noon after the murk of his dreamscape. His pulse is still pounding deafeningly inside his skull, making him feel dizzy as the panic refuses to recede.

There’s an unfamiliar pressure curled over his arm. He looks down and follows the drape of a dark appendage up to a familiar shadowy figure. Nightmare is standing over him, his single eye appraising Cross with some indecipherable emotion.

“The bed exists for a reason,” Nightmare says mildly. “You’re not meant to sleep on the floor.”

“Wasn’t trying to sleep,” Cross replies irritably, annoyed with himself. This is exactly the reason he hadn’t gotten on the bed. He’d hoped that sitting with his back against the unforgiving frame would be uncomfortable enough that he’d only be tempted to rest his eyes for a few short minutes. Evidently, he’d failed, and now there’s a painful knot between his spine and his scapula to punish him for his attempt. 

There’s no surprise at finding Nightmare in his room even though the door is locked. Cross has never let himself forget after that once time Horror came looking for a midnight snack. There’s wards in place to keep any of the others out, but this is Nightmare’s home and he goes where he pleases. It’s hardly a transgression when he’s the master of not only the castle but the entire world it’s built in. 

The tentacle draped on Cross’s shoulder winds around his nape and curls beneath his chin, guiding Cross to look up. Nightmare scrutinises him with care. “Is it time again already?”

Cross flinches, ready to instinctively deny it before forcibly biting his tongue. Even if he phrased it as a question, Nightmare undoubtedly knows much better than he does what’s needed, and Cross’s traumatic dreams are a telling symptom. He nods with slow reluctance.

“Very well,” Nightmare says, smiling as if pleased with Cross’s lack of argument. When Cross moves to get up, the tentacle on his shoulders bears down with slightly more force. “Stay as you are. We can begin now.”

Cross nods again. It’s a sensible suggestion, since he won’t be able to stand by the end anyway, but it feels too disrespectful to sit casually with his legs splayed out. That was more like something Killer would do. For his own peace of mind, he shifts onto his knees, instantly feeling more grounded and appropriately deferential. It’s more familiar too, emulating the previous occasions Nightmare has done this for him.

His soul is still pounding, probably because of Nightmare’s nearness. Despite his best efforts, his instincts always mistake the press of Nightmare’s aura as an attack and his presence as a threat. Only training and willpower let him sit in a semblance of calm instead of reaching for his knives, especially when the rest of Nightmare’s tentacles reach out to envelop him in their terrifying embrace.

One wraps firmly around his chest, uncomfortably reminiscent of the chains in his dream. Another curls low on his hips, bracing his lower back. The third guides his wrists together, lacing between them. Cross determinedly quells his urge to resist and lets himself be bound, reminding himself that it’s just a precaution, and one that wouldn’t be necessary if he could properly keep himself in check. 

It helps that, unlike the phantom memory of Chara’s bindings, Nightmare’s tentacles are soft and supple instead of hard and pinching. They undulate against him, a gentle pressure that’s almost like a caress. Despite the uneasy throb of his soulbeat, he lets himself lean into their powerful hold. The serpentine limbs always feel slightly cool against his own LV-fuelled heat, and despite their viscous appearance they never drip or stain. Their oozing shape remains cohesive, slippery and frictionless without being wet.

The first tentacle completes its coil around his neck and rises up to tap politely on his teeth.

“Open,” Nightmare says with just enough edge to make it an order. That’s easier for Cross to take than an offer. Commands means he doesn’t have to think or second-guess. There’s only one right answer, and that’s to part his teeth and let the tentacle slither its way into his mouth.

The first taste of it is always unpleasant. The outer layer of Nightmare’s form burns on his tongue like battery acid; sharp, sour and almost painful. The nerves of his tongue shriek and sizzle, filling his skull with an acrid bitterness, his taste buds shorting out like fuses in an overcharged circuit. The squirming length of the tentacle muffles the low grunt of his discomfort, but Nightmare must feel the vibration of the sound in his throat. His gaze seems to soften slightly, one hand reaching out to graze over the top of Cross’s skull. That brief offering of real contact, bone on bone, makes Cross shudder, and he quickly shuts his sockets to hide his humiliating gratitude from Nightmare’s watchful gaze. 

With his eyes closed, he’s more aware of how the tentacles are starting to work their way under his clothes. The one around his chest has moved to take up occupancy in his rib cage, applying a firm, gratifying pressure under his sternum. The one at his hips is moving more cautiously, its questing tip tracing across the crests of his illium as it circles his pelvic rim. Nightmare has told him that all the physical contact is an invaluable aid to his focus, helping him direct his magic more accurately. The discomforting physical intimacy is an almost negligible corollary considering it’s his soul Nightmare’s going to be influencing. 

And the shameful truth is that despite their constricting grip, some part of him relishes their touch. Nightmare’s aura prickles over him like static, but that just makes his presence so much more tangible, undeniable. The empty void of Cross’s world and the time he spent in it makes distance and isolation terrifying, but like this there’s no escaping the visceral reality of Nightmare’s being.

Or the immediacy of his touch, which Cross can’t stop his body from reacting to despite how completely inappropriate it is. The first time he’d fallen over himself with apologies, utterly mortified, full of shame. Nightmare had only assured him that it wasn’t a problem; it wouldn’t stop him from what he needed to do, and he won’t hold those unintended responses against Cross.

Even so, Cross feels a flush of embarrassment as his bones grow warmer, tingling wherever the tentacles make contact. He can’t stop himself from squirming, and Nightmare’s tentacles tighten in response.

“Relax,” he says, an unyielding demand that somehow works when Cross’s own efforts fail. His helpless twitching eases, and he’s rewarded with another caress across his temple. “That’s right. Let me in.”

All resistance is draining out of him like water through a sieve. He can’t hold it, not when it’s easier to let himself be shaped as Nightmare’s instrument. The tentacle in his mouth wedges itself deeper inside him, forcing his jaw to stretch accommodatingly around its thicker base. His tongue has gone numb to its caustic texture. All he can feel is a lingering thrum of pressure that feels strangely pleasant. His whole skull is buzzing with it, resonating with the same frequency. The tentacle bears down firmly against the back of his throat, a choking clot that makes Cross keen unhappily, but after a few seconds he feels a painless rupture and the appendage passes through the barrier of his magic to fill up his skull. 

The feeling is indescribable. There’s a weight behind his eyes that nearly feels like the onset of a migraine, but instead of excruciating agony there’s only a foggy euphoria. Distantly, Cross hears the shredded sounds he’s making, a litany of wet, obscene moans, but he no longer has the capacity to care about shame. 

He blinks slowly, feeling his eyelights fuzz and flicker as Nightmare’s tentacle moves behind his eye sockets. One eye goes blind entirely as the tip of the appendage wriggles out through the hole and back into the open. Nightmare greets it with a crooked finger, stroking the tip like it’s a favoured pet. He looks inordinately pleased, and Cross feels a strange echo of that satisfaction as if it’s his own.

“You’re mine now.”

Cross hears the words, but their meaning doesn’t penetrate, only the tone which is full of Nightmare’s silken approval. In the face of it, all he can do is surrender, and without a conscious thought his soul projects yearningly out from his chest. Through blurring vision Cross can make out its mis-matched shape. Chara’s half of the soul is blazing crimson like an angry bonfire, fluttering in the strange two-beat rhythm that humans have. His own half is silver-white and dripping, fat beads of pearlescent fluid trickling obscenely into Nightmare’s outstretched hands. Its own pulse is slow, almost lethargic, throbbing in tempo with his pelvis where he can feel the dense shape of another tentacle moving through him. 

Nightmare is everywhere, in each crevice and cavity, in his head, in his thoughts. It's only natural for his soul to place itself trustingly in Nightmare’s hands, letting itself be claimed.

No matter how hard he tries, Cross can never remember what happens once Nightmare touches his soul. Nightmare has explained that he needs Cross’s magic to weaken enough that his own will supersede it, and that awareness and memory are easily lost in that state, but it’s frustrating nonetheless. He wishes he had a better understanding of exactly how Nightmare keeps Chara locked away so that he’s no longer a malicious ghost in Cross’s mind so they didn’t have to renew the seals so often, though Nightmare claims not to mind. 

All he has are faint impressions, so nebulous he can’t be sure they’re not just delusions he’s conjured up to fill the blanks. He sometimes remembers a painful pinch, like a band tightening over his soul to separate the two halves. That part might be real because when he pulls his soul out between sessions he can see a thin dark line between its crooked seam, the residue of Nightmare’s influence.

At most he gets brief flashes of sensations: the soft, plush feeling that might be Nightmare’s tentacle cradling his soul, or a similar sensation that’s both hotter and wetter, reminding him more of a tongue. Sometimes it’s a stronger, more rigid pressure, like the tip of a finger gently tracing the swollen surface, stroking, pleasuring and persistent.

But it’s all vague and inconclusive. He only truly comes back to himself hours later, blinking awake with his body pleasantly aching and his soul blessedly silent. Glancing over himself, he sees he’s lying on the bed atop the blanket. His shoes have been removed, but otherwise his clothes are back in place with barely even a crease. There’s no evidence of Nightmare’s touch on him except the lingering imprint on his soul.

Nightmare is waiting for him. He always stays until Cross regains consciousness, an unexpected kindness. The moment he sees Cross looking back at him, he smiles.

“You should sleep easier now,” he says, which tells Cross all he needs to know. His body and soul are safe from Chara’s influence. “Try and get some proper rest this time.”

And then he’s gone, vanishing from Cross’s room like a phantom, disappearing into the shadows beneath the bed. For a moment, Cross almost feels bereft, untethered, but then the flicker of his disappointment tugs at the stitch of darkness across his soul. Oddly, it soothes him, making it easy to lay his head back down and do his best to follow Nightmare’s command. 


End file.
